the concept behind a "switching" power supply is that it generates the other voltages from the 5 volt load. so without a proper load it'll read incorrectly as you've discovered.
I don't really concern myself with +12 or -5 unless there's some kind of issue with the sound hardware. +12 has a wider tolerance than +5, it's not wildly uncommon to find this in the 11-14V range. for the record, I've never seen a situation where the voltage on the +12 line was so low that the amplifier wouldn't power up.
the above noted #555 lamp that I hook up is only really intended to pull the +5 within a safe operating range (after wiping the +5 adjustment), I always fine tune the voltage adjustment according to what I pull at game logic regardless of what I came up with having the lamp attached. what people generally neglect is that every cabinet is different and the wiring or integrity of header pins or edge connectors will dictate how much voltage flows through to the hardware.
that was the intention of the PSU guide in my signature.